Re: [PATCH Kernel v20 6/8] vfio iommu: Update UNMAP_DMA ioctl to get dirty bitmap before unmap

Kirti Wankhede
2020-05-15T15:30:48Z

On 5/15/2020 7:01 PM, Alex Williamson wrote:
> On Fri, 15 May 2020 12:17:03 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
>
>> On 5/15/2020 11:17 AM, Alex Williamson wrote:
>>> On Fri, 15 May 2020 09:46:43 +0530
>>> Kirti Wankhede <kwankhede@nvidia.com> wrote:
>>>
>>>> On 5/15/2020 8:57 AM, Alex Williamson wrote:
>>>>> On Fri, 15 May 2020 02:07:45 +0530
>>>>> Kirti Wankhede <kwankhede@nvidia.com> wrote:
>>>>>
>>>>>> DMA mapped pages, including those pinned by mdev vendor drivers, might
>>>>>> get unpinned and unmapped while migration is active and device is still
>>>>>> running. For example, in pre-copy phase while guest driver could access
>>>>>> those pages, host device or vendor driver can dirty these mapped pages.
>>>>>> Such pages should be marked dirty so as to maintain memory consistency
>>>>>> for a user making use of dirty page tracking.
>>>>>>
>>>>>> To get bitmap during unmap, user should allocate memory for bitmap, set
>>>>>> it all zeros, set size of allocated memory, set page size to be
>>>>>> considered for bitmap and set flag VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP.
>>>>>>
>>>>>> Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
>>>>>> Reviewed-by: Neo Jia <cjia@nvidia.com>
>>>>>> ---
>>>>>> drivers/vfio/vfio_iommu_type1.c | 77 ++++++++++++++++++++++++++++++++++-------
>>>>>> include/uapi/linux/vfio.h | 10 ++++++
>>>>>> 2 files changed, 75 insertions(+), 12 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
>>>>>> index b76d3b14abfd..a1dc57bcece5 100644
>>>>>> --- a/drivers/vfio/vfio_iommu_type1.c
>>>>>> +++ b/drivers/vfio/vfio_iommu_type1.c
>>>>>> @@ -195,11 +195,15 @@ static void vfio_unlink_dma(struct vfio_iommu *iommu, struct vfio_dma *old)
>>>>>> static int vfio_dma_bitmap_alloc(struct vfio_dma *dma, size_t pgsize)
>>>>>> {
>>>>>> uint64_t npages = dma->size / pgsize;
>>>>>> + size_t bitmap_size;
>>>>>>
>>>>>> if (npages > DIRTY_BITMAP_PAGES_MAX)
>>>>>> return -EINVAL;
>>>>>>
>>>>>> - dma->bitmap = kvzalloc(DIRTY_BITMAP_BYTES(npages), GFP_KERNEL);
>>>>>> + /* Allocate extra 64 bits which are used for bitmap manipulation */
>>>>>> + bitmap_size = DIRTY_BITMAP_BYTES(npages) + sizeof(u64);
>>>>>> +
>>>>>> + dma->bitmap = kvzalloc(bitmap_size, GFP_KERNEL);
>>>>>> if (!dma->bitmap)
>>>>>> return -ENOMEM;
>>>>>>
>>>>>> @@ -999,23 +1003,25 @@ static int verify_bitmap_size(uint64_t npages, uint64_t bitmap_size)
>>>>>> }
>>>>>>
>>>>>> static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>>>>>> - struct vfio_iommu_type1_dma_unmap *unmap)
>>>>>> + struct vfio_iommu_type1_dma_unmap *unmap,
>>>>>> + struct vfio_bitmap *bitmap)
>>>>>> {
>>>>>> - uint64_t mask;
>>>>>> struct vfio_dma *dma, *dma_last = NULL;
>>>>>> - size_t unmapped = 0;
>>>>>> + size_t unmapped = 0, pgsize;
>>>>>> int ret = 0, retries = 0;
>>>>>> + unsigned long pgshift;
>>>>>>
>>>>>> mutex_lock(&iommu->lock);
>>>>>>
>>>>>> - mask = ((uint64_t)1 << __ffs(iommu->pgsize_bitmap)) - 1;
>>>>>> + pgshift = __ffs(iommu->pgsize_bitmap);
>>>>>> + pgsize = (size_t)1 << pgshift;
>>>>>>
>>>>>> - if (unmap->iova & mask) {
>>>>>> + if (unmap->iova & (pgsize - 1)) {
>>>>>> ret = -EINVAL;
>>>>>> goto unlock;
>>>>>> }
>>>>>>
>>>>>> - if (!unmap->size || unmap->size & mask) {
>>>>>> + if (!unmap->size || unmap->size & (pgsize - 1)) {
>>>>>> ret = -EINVAL;
>>>>>> goto unlock;
>>>>>> }
>>>>>> @@ -1026,9 +1032,15 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>>>>>> goto unlock;
>>>>>> }
>>>>>>
>>>>>> - WARN_ON(mask & PAGE_MASK);
>>>>>> -again:
>>>>>> + /* When dirty tracking is enabled, allow only min supported pgsize */
>>>>>> + if ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
>>>>>> + (!iommu->dirty_page_tracking || (bitmap->pgsize != pgsize))) {
>>>>>> + ret = -EINVAL;
>>>>>> + goto unlock;
>>>>>> + }
>>>>>>
>>>>>> + WARN_ON((pgsize - 1) & PAGE_MASK);
>>>>>> +again:
>>>>>> /*
>>>>>> * vfio-iommu-type1 (v1) - User mappings were coalesced together to
>>>>>> * avoid tracking individual mappings. This means that the granularity
>>>>>> @@ -1066,6 +1078,7 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>>>>>> ret = -EINVAL;
>>>>>> goto unlock;
>>>>>> }
>>>>>> +
>>>>>> dma = vfio_find_dma(iommu, unmap->iova + unmap->size - 1, 0);
>>>>>> if (dma && dma->iova + dma->size != unmap->iova + unmap->size) {
>>>>>> ret = -EINVAL;
>>>>>> @@ -1083,6 +1096,23 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
>>>>>> if (dma->task->mm != current->mm)
>>>>>> break;
>>>>>>
>>>>>> + if ((unmap->flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) &&
>>>>>> + (dma_last != dma)) {
>>>>>> +
>>>>>> + /*
>>>>>> + * mark all pages dirty if all pages are pinned and
>>>>>> + * mapped
>>>>>> + */
>>>>>> + if (dma->iommu_mapped)
>>>>>> + bitmap_set(dma->bitmap, 0,
>>>>>> + dma->size >> pgshift);
>>>>>
>>>>> Nit, all the callers of update_user_bitmap() precede the call with this
>>>>> identical operation, we should probably push it into the function to do
>>>>> it.
>>>>>
>>>>>> +
>>>>>> + ret = update_user_bitmap(bitmap->data, dma,
>>>>>> + unmap->iova, pgsize);
>>>>>> + if (ret)
>>>>>> + break;
>>>>>> + }
>>>>>> +
>>>>>
>>>>> As noted last time, the above is just busy work if pfn_list is not
>>>>> already empty. The entire code block above should be moved to after
>>>>> the block below. Thanks,
>>>>>
>>>>
>>>> pfn_list will be empty for IOMMU backed devices where all pages are
>>>> pinned and mapped,
>>>
>>> Unless we're making use of the selective dirtying introduced in patch
>>> 8/8 or the container is shared with non-IOMMU backed mdevs.
>>>
>>>> but those should be reported as dirty.
>>>
>>> I'm confused how that justifies or requires this ordering.
>>>
>>
>> 1. non IOMMU mdev device:
>> - vendor driver pins pages
>> - pfn_list is not empty
>> - device dma or write to pinned pages
>>
>> 2. IOMMU backed mdev device or vfio device, but smart driver which pins
>> required pages
>> - vendor driver pins pages
>> - pfn_list is not empty
>> - device dma or write to pinned pages
>>
>> 3. IOMMU backed mdev device or vfio device, driver is not smart
>> - pages are pinned and mapped during attach
>> - pfn_list is empty
>> - device dma or write to any of pinned pages
>>
>> For case 3, here this function does bitmap_set(dma->bitmap), that is
>> mark all pages dirty and then accordingly copy bitmap to user buffer.
>> Copying dma->bitmap logic remains same.
>
> What dependency does case 3 have on pfn_list? Why does it matter if
> that bitmap_set() occurs before or after we've drained pfn_list? How
> do we know we're in case 3 before we've looked at pfn_list?
>

If update_user_bitmap() is moved in if block like:
if (!RB_EMPTY_ROOT(&dma->pfn_list)) {
update_user_bitmap()
...
}

then update_user_bitmap() would never get called for case 3.

>>>> So moved it
>>>> back above empty pfn_list check.
>>>
>>> Sorry, it still doesn't make any sense to me, and with no discussion I
>>> can't differentiate ignored comments from discarded comments.
>>>
>>> Pages in the pfn_list contribute to the dirty bitmap when they're
>>> pinned, we don't depend on pfn_list when reporting the dirty bitmap
>>> except for re-populating pfn_list dirtied pages after the bitmap has
>>> been cleared. We're unmapping the dma, so that's not the case here.
>>> Also since update_user_bitmap() shifts the bitmap in place now, any
>>> repetitive calls will give us incorrect results.
>>
>> Right, but this is unmapping and freeing vfio_dma
>>
>>> Therefore, as I see
>>> it, we _can_ take the branch below and when we do any work we've done
>>> above is not only wasted but may lead to incorrect data copied to
>>> the user if we shift dma->bitmap in place more than once. Please
>>> explain in more detail if you believe this is still correct. Thanks,
>>>
>>
>> In this case also bitmap copy to user happens once, (dma_last != dma)
>> takes care of making sure that its called only once.
>
> I did miss the dma_last check, so that prevents us from repeating this
> path, BUT we release iommu->lock if we enter the pfn_list !empty path.
> The moment we do that, we might unblock a vendor driver trying to pin
> or rw more pages. So the bitmap we've copied to the user might be stale
> and incorrect. I don't see why this is so contentious, what is the
> actual disadvantage to moving this until after we've drained pfn_list?
> The risks seem abundant to me. Thanks,
>

Sorry if I misinterpreted earlier, are you proposing to move
update_user_bitmap() after this if block?

if (!RB_EMPTY_ROOT(&dma->pfn_list)) {
...
}

I thought you are asking to move it in block.
That makes sense. I'll update the patch in send next version in some time.

Thanks,
Kirti

> Alex
>
>>>>>
>>>>>> if (!RB_EMPTY_ROOT(&dma->pfn_list)) {
>>>>>> struct vfio_iommu_type1_dma_unmap nb_unmap;
>>>>>>
>>>>>> @@ -2447,17 +2477,40 @@ static long vfio_iommu_type1_ioctl(void *iommu_data,
>>>>>>
>>>>>> } else if (cmd == VFIO_IOMMU_UNMAP_DMA) {
>>>>>> struct vfio_iommu_type1_dma_unmap unmap;
>>>>>> - long ret;
>>>>>> + struct vfio_bitmap bitmap = { 0 };
>>>>>> + int ret;
>>>>>>
>>>>>> minsz = offsetofend(struct vfio_iommu_type1_dma_unmap, size);
>>>>>>
>>>>>> if (copy_from_user(&unmap, (void __user *)arg, minsz))
>>>>>> return -EFAULT;
>>>>>>
>>>>>> - if (unmap.argsz < minsz || unmap.flags)
>>>>>> + if (unmap.argsz < minsz ||
>>>>>> + unmap.flags & ~VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP)
>>>>>> return -EINVAL;
>>>>>>
>>>>>> - ret = vfio_dma_do_unmap(iommu, &unmap);
>>>>>> + if (unmap.flags & VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP) {
>>>>>> + unsigned long pgshift;
>>>>>> +
>>>>>> + if (unmap.argsz < (minsz + sizeof(bitmap)))
>>>>>> + return -EINVAL;
>>>>>> +
>>>>>> + if (copy_from_user(&bitmap,
>>>>>> + (void __user *)(arg + minsz),
>>>>>> + sizeof(bitmap)))
>>>>>> + return -EFAULT;
>>>>>> +
>>>>>> + if (!access_ok((void __user *)bitmap.data, bitmap.size))
>>>>>> + return -EINVAL;
>>>>>> +
>>>>>> + pgshift = __ffs(bitmap.pgsize);
>>>>>> + ret = verify_bitmap_size(unmap.size >> pgshift,
>>>>>> + bitmap.size);
>>>>>> + if (ret)
>>>>>> + return ret;
>>>>>> + }
>>>>>> +
>>>>>> + ret = vfio_dma_do_unmap(iommu, &unmap, &bitmap);
>>>>>> if (ret)
>>>>>> return ret;
>>>>>>
>>>>>> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
>>>>>> index 123de3bc2dce..0a0c7315ddd6 100644
>>>>>> --- a/include/uapi/linux/vfio.h
>>>>>> +++ b/include/uapi/linux/vfio.h
>>>>>> @@ -1048,12 +1048,22 @@ struct vfio_bitmap {
>>>>>> * field. No guarantee is made to the user that arbitrary unmaps of iova
>>>>>> * or size different from those used in the original mapping call will
>>>>>> * succeed.
>>>>>> + * VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP should be set to get dirty bitmap
>>>>>> + * before unmapping IO virtual addresses. When this flag is set, user must
>>>>>> + * provide data[] as structure vfio_bitmap. User must allocate memory to get
>>>>>> + * bitmap, zero the bitmap memory and must set size of allocated memory in
>>>>>> + * vfio_bitmap.size field. A bit in bitmap represents one page of user provided
>>>>>> + * page size in 'pgsize', consecutively starting from iova offset. Bit set
>>>>>> + * indicates page at that offset from iova is dirty. Bitmap of pages in the
>>>>>> + * range of unmapped size is returned in vfio_bitmap.data
>>>>>> */
>>>>>> struct vfio_iommu_type1_dma_unmap {
>>>>>> __u32 argsz;
>>>>>> __u32 flags;
>>>>>> +#define VFIO_DMA_UNMAP_FLAG_GET_DIRTY_BITMAP (1 << 0)
>>>>>> __u64 iova; /* IO virtual address */
>>>>>> __u64 size; /* Size of mapping (bytes) */
>>>>>> + __u8 data[];
>>>>>> };
>>>>>>
>>>>>> #define VFIO_IOMMU_UNMAP_DMA _IO(VFIO_TYPE, VFIO_BASE + 14)
>>>>>
>>>>
>>>
>>
>